MR Image Quality Research Articles

WE‐G‐BRD‐09: Novel MRI Compatible Electron Accelerator for MRI‐Linac Radiotherapy

Purpose:MRI guided radiotherapy is a rapidly growing field; however current linacs are not designed to operate in MRI fringe fields. As such, current MRI‐ Linac systems require magnetic shielding, impairing MR image quality and system flexibility. Here, we present a bespoke electron accelerator concept with robust operation in in‐line magnetic fields.Methods:For in‐line MRI‐Linac systems, electron gun performance is the major constraint on accelerator performance. To overcome this, we propose placing a cathode directly within the first accelerating cavity. Such a configuration is used extensively in high energy particle physics, but not previously for radiotherapy. Benchmarked computational modelling (CST, Darmstadt, Germany) was employed to design and assess a 5.5 cell side coupled accelerator with a temperature limited thermionic cathode in the first accelerating cell. This simulation was coupled to magnetic fields from a 1T MRI model to assess robustness in magnetic fields for Source to Isocenter Distance between 1 and 2 meters. Performance was compared to a conventional electron gun based system in the same magnetic field.Results:A temperature limited cathode (work function 1.8eV, temperature 1245K, emission constant 60A/K/cm2) will emit a mean current density of 24mA/mm2 (Richardson's Law). We modeled a circular cathode with radius 2mm and mean current 300mA. Capture efficiency of the device was 43%, resulting in target current of 130 mA. The electron beam had a FWHM of 0.2mm, and mean energy of 5.9MeV (interquartile spread of 0.1MeV). Such an electron beam is suitable for radiotherapy, comparing favourably to conventional systems. This model was robust to operation the MRI fringe field, with a maximum current loss of 6% compared to 85% for the conventional system.Conclusion:The bespoke electron accelerator is robust to operation in in‐line magnetic fields. This will enable MRI‐Linacs with no accelerator magnetic shielding, and minimise painstaking optimisation of the MRI fringe field.This work was supported by US (NIH) and Australian (NHMRC & Cancer Institute NSW) government research funding. In addition, I would like to thank cancer institute NSW and the Ingham Institute for scholarship support.

TU-F-CAMPUS-I-04: Head-Only Asymmetric Gradient System Evaluation: ACR Image Quality and Acoustic Noise

Purpose: A high-performance head-only magnetic resonance imaging gradient system with an acquisition volume of 26 cm employing an asymmetric design for the transverse coils has been developed. It is able to reach a magnitude of 85 mT/m at a slew rate of 700 T/m/s, but operated at 80 mT/m and 500 T/m/s for this test. A challenge resulting from this asymmetric design is that the gradient nonlinearly exhibits both odd- and even-ordered terms, and as the full imaging field of view is often used, the nonlinearity is pronounced. The purpose of this work is to show the system can produce clinically useful images after an on-site gradient nonlinearity calibration and correction, and show that acoustic noise levels fall within non-significant risk (NSR) limits for standard clinical pulse sequences. Methods: The head-only gradient system was inserted into a standard 3T wide-bore scanner without acoustic damping. The ACR phantom was scanned in an 8-channel receive-only head coil and the standard American College of Radiology (ACR) MRI quality control (QC) test was performed. Acoustic noise levels were measured for several standard pulse sequences. Results: Images acquired with the head-only gradient system passed all ACR MR image quality tests; Both even and odd-order gradient distortion correction terms were required for the asymmetric gradients to pass. Acoustic noise measurements were within FDA NSR guidelines of 99 dBA (with assumed 20 dBA hearing protection) A-weighted and 140 dB for peak for all but one sequence. Note the gradient system was installed without any shroud or acoustic batting. We expect final system integration to greatly reduce noise experienced by the patient. Conclusion: A high-performance head-only asymmetric gradient system operating at 80 mT/m and 500 T/m/s conforms to FDA acoustic noise limits in all but one case, and passes all the ACR MR image quality control tests. This work was supported in part by the NIH grant 5R01EB010065

SU‐E‐T‐661: Quantitative MRI Assessment of a Novel Direction‐Modulated Brachytherapy Tandem Applicator for Cervical Cancer

Purpose:To quantitatively evaluate the MR image quality of a novel direction modulated brachytherapy (DMBT) tandem applicator for cervical cancer, using the clinical MRI scanning protocol for image guided brachytherapy.Methods:The tungsten alloy‐based applicator was placed in a water phantom and clinical imaging protocol was performed. Axial images were acquired using 2D turbo‐spin echo (TSE) T2‐weighted sequence on a 1.5T GE 450w MR scanner and an 8‐channel body coil. As multi‐channel receiver coil was used, inhomogeneities in the B1 receive field must be considered before performing the quantification process. Therefore the applicator was removed from the phantom and the whole imaging session was performed again for the water phantom with the same parameters. Images from the two scans were then subtracted, resulting in a difference image that only shows the applicator with its surrounding magnetic susceptibility dipole artifact. Line profiles were drawn and plotted on the difference image at various angles and locations along the tandem. Full width at half maximum (FWHM) was measured at all the line profiles to quantify the extent of the artifact. Additionally, the extent of the artifact along the diameter of the tandem was measured at various angles and locations.Results:After removing the background inhomogeneities of the receiver coil, FWHM of the tandem measured 5.75 ± 0.35 mm (the physical tandem diameter is 5.4 mm). The average extent of the artifacts along the diameter of the tandem measured is 2.14 ± 0.56 mm. In contrast to CT imaging of the same applicator (not shown here), the tandem can be easily identified without additional correction algorithms.Conclusion:This work demonstrated that the novel DMBT tandem applicator has minimal susceptibility artifact in T2‐weighted images employed in clinical practice for MRI‐guided brachytherapy of cervical cancer.

Long-term stability of the MR system of the Philips Ingenuity TF

The Philips Ingenuity TF is a sequential PET/MR with an Achieva 3T X-series MR and Gemini TF PET system connected with a rotating table. Both the PET and MR systems are designed to minimize mutual system interference. However, a longitudinal study concerning the stability of the MR system has not been conducted before. The stability of the MR system affects diffusion weighted imaging (DWI), diffusion tensor imaging (DTI) and functional MRI (fMRI), especially in longterm and multi-center studies. In addition, the variation of geometric distortions and consistency of MR image quality is important for clinical studies and radiotherapy planning. The long-term stability of the MR system was monitored by weekly quality control (QC) scans during one year. For measuring DWI, DTI and fMRI stability, protocols from fBIRN and vendor QA were implemented by using a spherical agar phantom and a liquid sphere phantom. For measuring the variations of image quality, ACR phantom measurements were performed. For the fBIRN and vendor QA protocols, automatic QC programs provided by the consortium and manufacturer were used. For the ACR phantom, an automatic quality control measurement was developed in MATLAB 2011b. Analysis of the QC scans did not show any significant variations with the fBIRN, vendor QA or ACR protocols. Thus, the MR system of the Ingenuity TF was deemed to be stable in the longitudinal study. Automated analysis of QC measurements was deemed necessary for implementing multiple, weekly measurements for PET/MR.

A resistive heating system for homeothermic maintenance in small animals

PurposeTo develop an MR-compatible resistive heater for temperature maintenance of anaesthetized animals. Materials and MethodsAn MR-compatible resistive electrical heater was formed from a tightly-wound twisted pair wire, interfaced to a homeothermic maintenance controller. Fat-suppressed images and localized spectra were acquired with the twisted pair heater and a near-identical single strand heater during operation at maximum power. Data were also acquired in the absence of heating to demonstrate the insensitivity of MR to distortions arising from the passage of current through the heater elements. The efficacy of temperature maintenance was examined by measuring rectal temperature immediately following induction of general anesthesia and throughout and after the acquisition of a heater artifact-prone image series. ResultsImages and spectra acquired in the presence and absence of DC current through the twisted pair heater were identical whereas the passage of current through the single strand wire created field shifts and lineshape distortions. Temperature that is lost during anesthesia induction was recovered within approximately 10–20 minutes of induction, and a stable temperature is reached as the animal's temperature approaches the set target. ConclusionThe twisted pair wire heater does not interfere with MR image quality and maintains adequate thermal input to the animal to maintain body temperature.

Feasibility and evaluation of dual-source transmit 3D imaging of the orbits: Comparison to high-resolution conventional MRI at 3T

PurposeTo prospectively compare the image quality and diagnostic performance of orbital MR images obtained by using a dual-source parallel transmission (pTX) 3D sequence (Sampling Perfection with Application optimized Contrasts using different flip angle Evolution, SPACE) with the image quality of conventional high-resolution standard protocol for clinical use in patients at 3T. Materials and methodsAfter obtaining institutional review board approval and patient consent, 32 patients with clinical indication for orbital MRI were examined using a high-resolution conventional sequences and 3D pTX SPACE sequences. Quantitative measurements, image quality of the healthy orbit, incidence of artifacts, and the subjective diagnostic performance to establish diagnosis was rated. Statistical significance was calculated by using a Student's t-test and nonparametric Wilcoxon signed rank test. ResultsLength measurements were comparable in the two techniques, 3D pTX SPACE resulted in significant faster image acquisition with higher spatial resolution and less motion artifacts as well as better delineation of the optic nerve sheath. However, estimated contrast-to-noise and signal-to-noise and overall image quality as well as subjective scores of the conventional TSE imaging were rated significantly higher. The conventional MR sequences were the preferred techniques by the readers. ConclusionThis study demonstrates the feasibility of 3D pTX SPACE of the orbit resulting in a rapid acquisition of isotropic high-resolution images. Although no pathology was missed in 3D pTX SPACE, conventional MRI techniques showed the higher diagnostic confidence in our study, presumably due to the higher signal-to-noise and contrast-to-noise ratios. We observed high-resolution TSE imaging to be the preferred technique, 3D pTX SPACE cannot replace conventional MRI so far.

Cardiac MRI in patients with cardiac pacemakers: practical methods for reducing susceptibility artifacts and optimizing image quality.

Magnetic resonance imaging (MRI) of pacemaker patients has become available despite of previous contraindications. However, pacing systems containing ferromagnetic material may hamper the diagnostic quality of cardiac MR (CMR) images. To study methods for reducing susceptibility-based artifacts in CMR examinations of pacemaker patients. Altogether 16 patients were scanned with 1.5T MRI scanner using cine balanced steady-state free-precession (bSSFP) and spoiled gradient echo (SPGR) sequences. The use of frequency-scout was also evaluated. For myocardial late gadolinium-enhanced (LGE) imaging, SPGR or bSSFP readout inversion-recovery prepared gradient echo sequences were used with and without phase-sensitive inversion-recovery (PSIR). Two radiologists subjectively compared the image quality (IQ) and the ranges of susceptibility artifacts were evaluated objectively. The IQ proved adequate for diagnosing each patient, although in a few patients with a left-side implanted generator, artifacts hampered IQ in the anterior and anteroseptal segments of the myocardium in bSSFP cine and LGE sequences. In bSSFP cine, the use of frequency-scout could often transfer the banding artifacts away from the left ventricular myocardium. In LGE imaging, the artifacts were more pronounced in IR-bSSFP and PSIR than in IR-SPGR sequences. The ranges of generator-based artifacts were greater in bSSFP (10-12 cm) than in SPGR (6 cm) sequences due to banding artifacts. The artifacts caused by pacemakers typically did not compromise the diagnostic IQ. The use of frequency-scout prior to bSSFP cine or the use of SPGR-based sequences could also improve IQ.

Knee MRI under varying flexion angles utilizing a flexible flat cable antenna.

The aim of this study is to fabricate and test a novel flexible flat cable antenna (FFCA) for MRI of the knee at different flexion angles. The FFCA was made of a flat cable, a tuning/matching circuit and a signal transmission line. To test its feasibility and validity, in vitro and in vivo experiments were carried out on a 3.0 T MR scanner. The in vitro experiment suggested that the proposed FFCA could achieve a high signal-to-noise ratio (SNR) of 336, while the SNR of an eight-channel knee coil was 291, and phantom images from the FFCA are homogeneously distributed. In the in vivo experiment, the FFCA had a higher SNR of 169 in the region of interest and more than 48.5 cm of longitudinal coverage, while the corresponding values for the commercial coil were 153 and 22.5 cm. Finally, five sagittal knee images at different flexion angles were acquired. The FFCA could acquire satisfactory knee images at different flexion angles, with the advantages of simplicity, low cost, large field of view and high SNR. It may therefore be further used to improve MR image quality of the knee joint.

Application of texture analysis to muscle MRI: 2 – technical recommendations

A goal of the multicenter European Cooperation in Science and Technology (COST) action MYO-MRI is to optimize Magnetic Resonance Imaging Texture Analysis (MRI-TA) methods for application in the study of muscle disease. This paper deals with recommendations on the optimal methodology to collect the MRI data, to analyse it via texture analysis and to make conclusions from the resultant texture parameter data. A full and detailed description is provided with respect to MR image quality control, sequence choice, image pre-processing, region of interest selection, texture analysis methods and data analysis. A series of conclusions are presented.

The Kora Pacemaker is Safe and Effective for Magnetic Resonance Imaging.

BACKGROUNDThe impact of magnetic resonance imaging (MRI) on pacemakers is potentially hazardous. We present clinical results from a novel MRI conditional pacing system with the capability to switch automatically to asynchronous mode in the presence of a strong magnetic field.AIMSThe IKONE (Assessment of the MRI solution: KORA 100™ and Beflex™ pacing leads system) study is an open-label, prospective, multicenter study aimed at confirming the safety and effectiveness of the system, when used in patients undergoing MRI of anatomical regions excluding the chest.METHODSPrimary eligibility criteria included patients implanted with the system, with or without a clinically indicated MRI. The primary endpoint was to confirm no significant change in pacing capture thresholds at 1 month after an MRI, with an absolute difference of ≤0.75 V between the pre- and 1-month post-MRI for both atrial and ventricular capture thresholds.RESULTSOut of 33 patients enrolled (mean age: 72.8 ± 11.4 years, 70% male, implant indication or device), 29 patients implanted with the MRI conditional system underwent an MRI 6–8 week postimplant. The study reached its primary endpoint: the mean absolute difference in pacing capture threshold at 1-month post-MRI versus pre-MRI was less than 0.75 V in the atrium (Δ = 0.18 ± 0.16 V, P-value < 0.001) and in the ventricle (Δ = 0.18 ± 0.22 V, P-value < 0.001). There were no adverse events related to the MRI procedure nor were there reports of patient symptoms or discomfort associated. MR image quality was of diagnostic quality in all patients.CONCLUSIONLead electrical performance as measured by difference in capture thresholds were not impacted by MRI. This first clinical evaluation of a novel MRI conditional system demonstrates it is safe and effective for use in out-of-chest, 1.5-T MR imaging.

A 16-channel MR coil for simultaneous PET/MR imaging in breast cancer.

To implement and evaluate a dedicated receiver array coil for simultaneous positron emission tomography/magnetic resonance (PET/MR) imaging in breast cancer. A 16-channel receiver coil design was optimized for simultaneous PET/MR imaging. To assess MR performance, the signal-to-noise ratio, parallel imaging capability and image quality was evaluated in phantoms, volunteers and patients and compared to clinical standard protocols. For PET evaluation, quantitative (18) F-FDG PET images of phantoms and seven patients (14 lesions) were compared to images without the coil. In PET image reconstruction, a CT-based template of the coil was combined with the MR-acquired attenuation correction (AC) map of the phantom/patient. MR image quality was comparable to clinical MR-only examinations. PET evaluation in phantoms showed regionally varying underestimation of the standardised uptake value (SUV; mean 22 %) due to attenuation caused by the coil. This was improved by implementing the CT-based coil template in the AC (<2 % SUV underestimation). Patient data indicated that including the coil in the AC increased the SUV values in the lesions (21 ± 9 %). Using a dedicated PET/MR breast coil, state-of-the-art MRI was possible. In PET, accurate quantification and image homogeneity could be achieved if a CT-template of this coil was included in the AC for PET image reconstruction. • State-of-the-art breast MRI using a dedicated PET/MR breast coil is feasible. • A multi-channel design facilitates shorter MR acquisition times via parallel imaging. • An MR coil inside a simultaneous PET/MR system causes PET photon attenuation. • Including a coil CT-template in PET image reconstruction results in recovering accurate quantification.

K-space sampling optimization for ultrashort TE imaging of cortical bone: applications in radiation therapy planning and MR-based PET attenuation correction.

The ultrashort echo-time (UTE) sequence is a promising MR pulse sequence for imaging cortical bone which is otherwise difficult to image using conventional MR sequences and also poses strong attenuation for photons in radiation therapy and PET imaging. The authors report here a systematic characterization of cortical bone signal decay and a scanning time optimization strategy for the UTE sequence through k-space undersampling, which can result in up to a 75% reduction in acquisition time. Using the undersampled UTE imaging sequence, the authors also attempted to quantitatively investigate the MR properties of cortical bone in healthy volunteers, thus demonstrating the feasibility of using such a technique for generating bone-enhanced images which can be used for radiation therapy planning and attenuation correction with PET/MR. An angularly undersampled, radially encoded UTE sequence was used for scanning the brains of healthy volunteers. Quantitative MR characterization of tissue properties, including water fraction and R2(∗) = 1/T2(∗), was performed by analyzing the UTE images acquired at multiple echo times. The impact of different sampling rates was evaluated through systematic comparison of the MR image quality, bone-enhanced image quality, image noise, water fraction, and R2(∗) of cortical bone. A reduced angular sampling rate of the UTE trajectory achieves acquisition durations in proportion to the sampling rate and in as short as 25% of the time required for full sampling using a standard Cartesian acquisition, while preserving unique MR contrast within the skull at the cost of a minimal increase in noise level. The R2(∗) of human skull was measured as 0.2-0.3 ms(-1) depending on the specific region, which is more than ten times greater than the R2(∗) of soft tissue. The water fraction in human skull was measured to be 60%-80%, which is significantly less than the >90% water fraction in brain. High-quality, bone-enhanced images can be generated using a reduced sampled UTE sequence with no visible compromise in image quality and they preserved bone-to-air contrast with as low as a 25% sampling rate. This UTE strategy with angular undersampling preserves the image quality and contrast of cortical bone, while reducing the total scanning time by as much as 75%. The quantitative results of R2(∗) and the water fraction of skull based on Dixon analysis of UTE images acquired at multiple echo times provide guidance for the clinical adoption and further parameter optimization of the UTE sequence when used for radiation therapy and MR-based PET attenuation correction.

Linear Dynamic Sparse Modelling for functional MR imaging.

The reconstruction quality of a functional MRI sequence is determined by reconstruction algorithms as well as the information obtained from measurements. In this paper, we propose a Linear Dynamic Sparse Modelling method which is composed of measurement design and reconstruction processes to improve the image quality from both aspects. This method models an fMRI sequence as a linear dynamic sparse model which is based on a key assumption that variations of functional MR images are sparse over time in the wavelet domain. The Hierarchical Bayesian Kalman filter which follows the model is employed to implement the reconstruction process. To accomplish the measurement design process, we propose an Informative Measurement Design (IMD) method. The IMD method addresses the measurement design problem of selecting k feasible measurements such that the mutual information between the unknown image and measurements is maximised, where k is a given budget and the mutual information is extracted from the linear dynamic sparse model. The experimental results demonstrated that our proposed method succeeded in boosting the quality of functional MR images.

An optimal RF shielding method for MR‐PET fusion system with insertable PET

ABSTRACTAn RF shielding method is proposed to preserve the resonance frequency of an RF coil regardless of the existence of a positron emission tomography (PET) detector module, so that the RF coil can effectively operate for both simultaneous MR‐PET imaging and MR stand‐alone imaging. The RF shield between the RF coil and the PET detector module was manufactured in the form of a hollow acryl cylinder wrapped with gold‐taffeta woven tape. As we adopted a double‐layer RF shield between the RF coil and the PET detector module, it was possible to maintain the resonance frequency of the RF coil and the MR image quality was similar in both cases, with and without the insertable PET detector module. Using the insertable concept of the PET system and the RF coil with an additional double‐layer RF shield, both an MR‐PET fusion system and an MR stand‐alone system were realized. © 2014 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 24, 263–269, 2014

Preliminary evaluation of image quality in a new clinical ToF-PET/MR scanner.

A new ToF-PET/MR scanner has recently been deployed at the University Hospital of Zurich for clinical evaluation purposes. The goal of the present work was to compare the image quality obtained on this system with that of equivalent state-of-the-art standalone scanners. The GE Signa PET/MR is a whole-body scanner combining a 3T wide-bore MR system with a 25 cm PET detector ring based on SiPM technology and mounted on a customized radiofrequency coil. MR image quality of this system was tested by scanning two volunteers: One with a brain protocol and another with a whole-body protocol. The same scans were repeated before and after PET detectors were installed on the system. A baseline measurement was acquired on a Discovery 750w scanner with the same volunteers. PET image quality was tested on patients referred for an FDG PET/CT scan. After 15-20 minutes of PET/CT, the patients were moved to the PET/MR, where an equivalent acquisition was performed. Data were reviewed by nuclear medicine and radiology physicians. As seen in Figure ​Figure1,1, MR image quality in the brain is identical between PET/MR (pre- and post-insert) and MR750w. In whole-body scans, shading on the images due to different receive coil sensitivity were observed. These are explained by the replacement of the bed-mounted posterior array coil by a central matrix array at the scanner iso-center. Figure ​Figure22 illustrates the differences in PET, showing equivalent or better quality in the PET/MR data, despite latter acquisition, justified by the increased sensitivity of the scanner. Figure 1 Coronal 2D MERGE views of a healthy volunteer scanned on a Discovery 750w (left), on the PET/MR before the installation of the PET insert (center) and on the same scanner once the insert was in place and online (right). Figure 2 Three-dimensional maximum intensity projection of a PET/CT acquisition (left) and posterior PET/MR acquisition (right) of the same patient. Our results show that the image quality of clinical MR sequences is not affected by the presence of the PET detector insert. PET image quality shows great promise, but will require further testing to confirm this quantitatively.

A 3D MR-acquisition scheme for non-rigid bulk motion correction in simultaneous PET-MR.

Positron emission tomography (PET) is commonly used to detect tumours and assess the progress of cancer treatment by measuring standardized uptake values (SUV). Physiological motion during data acquisition can negatively impair obtained SUV. Simultaneous PET-MR combines metabolic PET information with high-resolution anatomical MR images and has the potential to minimize motion artefacts. A 3D MR-acquisition scheme is proposed which allows for the automatic detection and correction of non-rigid bulk motion shifts, i.e. changes of the patient’s position leading to non-rigid changes of the patient’s anatomy, in PET and MR images (both T1-weighted gradient echo and T2-weighted turbo spin echo images) without an increase in scan time. Six respiratory gated T1- and T2-weighted 3D data sets with an isotropic resolution of 1.5mm were obtained using a radial phase encoding (RPE) acquisition. Volunteers are asked to move the abdomen during data acquisition resulting in overall 19 movements at arbitrary time points. RPE allows for a retrospective reconstruction of dynamic 3D MR images with different temporal resolutions from the same data. Non-rigid bulk motion is detected and corrected from these dynamic images. A simultaneous PET acquisition is simulated to assess the effect of motion correction on image quality and SUV for lesions with different diameters. Every bulk motion shift was accurately detected and corrected. Non-rigid motion fields describing the different motion states were obtained with an accuracy of 1.71 ± 0.29mm. The PET simulation showed SUV errors of up to 67% due to bulk motion which could be reduced to 10% with the proposed motion compensated approach. The presented MR acquisition scheme yields both high resolution 3D anatomical data and highly accurate non-rigid motion information without an increase in scan time. This method strongly improves both MR and PET image quality and ensures an accurate assessment of tracer uptake.

A 3D MR-acquisition scheme for nonrigid bulk motion correction in simultaneous PET-MR.

Positron emission tomography (PET) is a highly sensitive medical imaging technique commonly used to detect and assess tumor lesions. Magnetic resonance imaging (MRI) provides high resolution anatomical images with different contrasts and a range of additional information important for cancer diagnosis. Recently, simultaneous PET-MR systems have been released with the promise to provide complementary information from both modalities in a single examination. Due to long scan times, subject nonrigid bulk motion, i.e., changes of the patient's position on the scanner table leading to nonrigid changes of the patient's anatomy, during data acquisition can negatively impair image quality and tracer uptake quantification. A 3D MR-acquisition scheme is proposed to detect and correct for nonrigid bulk motion in simultaneously acquired PET-MR data. A respiratory navigated three dimensional (3D) MR-acquisition with Radial Phase Encoding (RPE) is used to obtain T1- and T2-weighted data with an isotropic resolution of 1.5 mm. Healthy volunteers are asked to move the abdomen two to three times during data acquisition resulting in overall 19 movements at arbitrary time points. The acquisition scheme is used to retrospectively reconstruct dynamic 3D MR images with different temporal resolutions. Nonrigid bulk motion is detected and corrected in this image data. A simultaneous PET acquisition is simulated and the effect of motion correction is assessed on image quality and standardized uptake values (SUV) for lesions with different diameters. Six respiratory gated 3D data sets with T1- and T2-weighted contrast have been obtained in healthy volunteers. All bulk motion shifts have successfully been detected and motion fields describing the transformation between the different motion states could be obtained with an accuracy of 1.71 ± 0.29 mm. The PET simulation showed errors of up to 67% in measured SUV due to bulk motion which could be reduced to less than 10% with the proposed motion compensation approach. A MR acquisition scheme which yields both high resolution 3D anatomical data and highly accurate nonrigid motion information without an increase in scan time is presented. The proposed method leads to a strong improvement in both MR and PET image quality and ensures an accurate assessment of tracer uptake.

Dynamic keyhole: a novel method to improve MR images in the presence of respiratory motion for real-time MRI.

In this work, the authors present a novel magnetic resonance imaging reconstruction method to improve the quality of MR images in the presence of respiratory motion for real-time thoracic image-guided radiotherapy. This new reconstruction method is called dynamic keyhole and utilizes a library of previously acquired, peripheral k-space datasets from the same (or similar) respiratory state in conjunction with central k-space datasets acquired in real-time. Internal or external respiratory signals are utilized to sort, match, and combine the two separate peripheral and central k-space datasets with respect to respiratory displacement, thereby reducing acquisition time and improving image quality without respiratory-related artifacts. In this study, the dynamic keyhole, conventional keyhole, and zero-filling methods were compared to full k-space acquisition (ground truth) for 60 coronal datasets acquired from 15 healthy human subjects. For the same image-quality difference from the ground-truth image, the dynamic keyhole method reused 79% of the prior peripheral phase-encoding lines, while the conventional keyhole reused 73% and zero-filling 63% (p-value < 0.0001), corresponding to faster acquisition speed of dynamic keyhole for real-time imaging applications. This study demonstrates that the dynamic keyhole method is a promising technique for clinical applications such as image-guided radiotherapy requiring real-time MR monitoring of the thoracic region. Based on the results from this study, the dynamic keyhole method could increase the temporal resolution by a factor of five compared with full k-space methods.

An MR-conditional high-torque pneumatic stepper motor for MRI-guided and robot-assisted intervention.

Magnetic resonance imaging allows for visualizing detailed pathological and morphological changes of soft tissue. MR-conditional actuations have been widely investigated for development of image-guided and robot-assisted surgical devices under the Magnetic resonance imaging (MRI). This paper presents a simple design of MR-conditional stepper motor which can provide precise and high-torque actuation without adversely affecting the MR image quality. This stepper motor consists of two MR-conditional pneumatic cylinders and the corresponding supporting structures. Alternating the pressurized air can drive the motor to rotate each step in 3.6° with the motor coupled to a planetary gearbox. Experimental studies were conducted to validate its dynamics performance. Maximum 800mNm output torque is achieved. The motor accuracy independently varied by two factors: motor operating speed and step size, was also investigated. The motor was tested within a 3T Siemens MRI scanner (MAGNETOM Skyra, Siemens Medical Solutions, Erlangen, Germany) and a 3T GE MRI scanner (GE SignaHDx, GE Healthcare, Milwaukee, WI, USA). The image artifact and the signal-to-noise ratio (SNR) were evaluated for study of its MRI compliancy. The results show that the presented pneumatic stepper motor generated 2.35% SNR reduction in MR images. No observable artifact was presented besides the motor body itself. The proposed motor test also demonstrates a standard to evaluate the pneumatic motor capability for later incorporation with motorized devices used under MRI.

Mechanistic force modeling and machinability evaluation on MR-compatible materials

Magnetic resonance imaging (MRI)-guided surgical robots are becoming a hotspot owing to the good quality of MR image and stability of robot. For the MR-compatible robot, machinability influences assembly accuracy and kinematic accuracy directly. At the same time, the cutting force plays a vital role in evaluating machinability and machining process. This paper presents the machinability evaluation on five kinds of MR-compatible materials and an improved mechanistic force model. The model takes shearing and edge forces of flank and bottom edge combined with tool runout into consideration. Experimental results show that the prediction error is within 9 %. The effects that chip shape imposes on the machining stability and surface roughness are analyzed and then involved in the machinability evaluation. Digraph and matrix method are applied to evaluate the material machinability based on cutting force, roughness, and the chip shape. Relative importance between the three components is analyzed and taken into the evaluation. At last, polyformaldehyde is proved to have the best machinability.